As the summer comes to an end, we are still enjoying a rare sight for this season: sunny and dry days. The rest of the summer was in contrast very wet and stormy. July is climatologically the driest month of the year in Boston with a normal total of about 3.06 in (78 mm). This year the accumulated rain during July was 6.00 in (152 mm) with a total of 17 days with rain making it the 6th wettest July since 1872. June and August so far have also been wetter than normal. The total seasonal rainfall (13.9 in) is far from record breaking (24.89 inches in 1955) but it is still remarkable, making it at least within the 15 percent of the rainiest summers in Boston.
New England has a few days in the year that we, people from more template climates, can properly call fall. These are those days, and they are the fair warning of the winter that comes ahead. Be quick to see the foliage as it will be a few weeks before snow comes in!
The remnants of tropical storm Hanna swept through Boston at a relatively innocuous timing (well, it depends for whom) and much weakened specifically in wind, so that it was hard to distinguish it from a regular rainstorm. After the passage of Hanna, weather has been fair and pleasant. Unfortunately this nice trend will end today abruptly as a cold front pushes through our region. The cold front will manifest itself with thunderstorms that will begin late in the afternoon. A high pressure will bring relatively cool weather with temperatures that might require you to dust off a sweater or a jacket for Wednesday evening.
Volcanic eruptions are natural phenomena ever present in the Earth’s history, although not in our minds most of the time. However, they are critical to the history and evolution of the Earth’s atmosphere. The atmosphere of the Earth before life had a similar composition to modern volcanic outgassing (mainly CO2, water and nitrogen), and all the water present in the oceans as well as most of the atmosphere is thought to have a volcanic origin. Volcanoes can influence climate in shorter time scales by injecting reflective sulfate aerosols and can also modify the chemical composition of the stratosphere influencing ozone depletion. The most spectacular case of volcanic eruption during the past century was the eruption of Mount Pinatubo in 1991, that is believed to have cooled the planet by about 0.5 C, an amount similar in magnitude to the accumulated trend in warming during the last 100 years.
As computers and models become more sophisticated, weather forecasting is growing increasingly independent of humans. Even the text of this analysis could eventually be produced by a computer. What then is the contribution of humans in the process of weather forecasting? If the contribution is merely communicating the forecast, then meteorology as a physical science is only important in providing the tools to improve models. People are not usually interested in the physical consistency of their forecasts, rather, they are interested in the forecasts themselves and their accuracy. Once a model is capable of producing a forecast, the communication of the forecast to the public proceeds independently from the physical principles on which the forecast was based.
We continue today under the influence of a very cold high pressure, the same system that has being giving us unseasonably cold temperatures over the last week. In fact, yesterday, we experienced temperatures that are normal for the beginning of January rather than early December.
The most interesting aspect of today’s forecast is the possibility that we will see firsthand some of the effects of Noel (officially a hurricane by yesterday at 8 p.m.). As the tropical storm transitions into an extratropical cyclone, it will weaken back to tropical storm strength, to later intensify into hurricane force winds by Saturday afternoon. By Saturday at 5 p.m., the center storm will (most likely) pass at about 150 miles from the coast of Massachusetts with some models (those with the westernmost trajectories) giving even hurricane force winds for the Cape and tropical storm winds for Boston. The exact trajectory of the storm is at this moment uncertain, but there is the potential of significant wind related disruptions for Saturday afternoon and evening, so plan your activities accordingly and stay tuned to the latest forecasts.
Yesterday started off very cold and damp, however it quickly shifted to sunny weather and temperatures slightly above 70°F. If you are curious about the source of this springtime variability, at least part of it is due to the jet stream that is currently located over Boston. The jet acts as a guide for smaller scale perturbations that can potentially cause weather variations over a few hours. Also, since the jet stream in the upper atmosphere is concomitant with a large temperature gradient near the surface and lower atmosphere, to be close to the jet is also to be close to the boundary between the polar air masses and the tropical air masses. This is yet another source of variability since it provides the potential for rapid changes in temperature due to air crossing over either from the pole or from the equator side of the boundary.
The reversal of seasons between hemispheres should not be a surprise to most educated people in the world (although we are reminded of a famous educational experiment in which Harvard graduates failed to give a good reason for why this happens). As we entered the autumn season in the northern hemisphere, the winter came to an end in the southern hemisphere. The sun being close to the equator this time of the year makes the mean temperatures similar across the same latitudes in both hemispheres. According to this simple rule, we should find that locations in the southern hemisphere around 42° of latitude have about the same temperatures as we are experiencing in Boston.
A very interesting article by K. Pennessi in the July issue of the <i>Bulletin of the American Meteorological Society</i> discusses some of the intricacies of the communication of meteorological forecasts (in the case of the article, it was the communication of seasonal forecast to Brazilian farmers).The author wanted to know what was deemed important to farmers when facing the official forecast agency as well as the prophecies of the local rain prophets.
Once strong, the low pressure centered over the Midwest is now an old storm, slowly filling up and loosing power. During the peak of its activity though,the low produced a potpourri of bad weather — tornadoes, hailstorms, and snow from Missouri to Illinois. The cold front, part of the low, will continue making its way towards the Atlantic, affecting Florida and the southern part of the East Coast with thunderstorms.